Early key telephone systems utilized separate pairs of lines for both the voice and signalling paths thereby necessitating upwards of forty or more individual conductors for the operation of a 6-button key station set. Utilizing electronic control combined with an intermediate switching network has enabled this to be reduced to a single pair of wires for the voice path and one or two additional pairs for power and control signalling. U.S. Pat. No. 3,549,820 of Dieter J. H. Knollman, issued Dec. 22, 1970 and entitled, "Key Telephone Station Concentrator" discloses an early system of this type. U.S. Pat. No. 3,647,980, of Lucian P. Fabiano, Jr., et al, issued Mar. 7, 1972 and entitled, "Switching Network Control without Network Map" discloses an improved central processor controlled key telephone system. More recently, computer controlled switching systems provide many features of the early key telephone systems plus more advanced features such as conferencing, call transferring, speed calling, and call forwarding.
In small installations, there is a need for many of the features offered by these systems while retaining a directly connected central office powered service which would continue to operate should power to the locally controlled key telephone network fail.